This invention relates to modulator or mixer circuits and related methods, and more particularly to Gilbert cell modulators and related methods.
Modulation systems and methods are widely used in transmitters to modulate information including voice and/or data onto a carrier. The carrier may be a final carrier or an intermediate carrier. The carrier frequency can be in UHF, VHF, RF, microwave or any other frequency band. Modulators are also referred to as xe2x80x9cmixersxe2x80x9d or xe2x80x9cmultipliersxe2x80x9d. For example, in a mobile terminal, a modulator may be used in the transmitter thereof, to modulate an input signal (voice and/or data) for wireless transmission.
A particular type of modulator which is widely used is the xe2x80x9cGilbert Multiplier Cellxe2x80x9d also referred to as the xe2x80x9cGilbert modulatorxe2x80x9d, the xe2x80x9cGilbert cellxe2x80x9d or the xe2x80x9cGilbert mixerxe2x80x9d. The Gilbert Multiplier Cell includes an emitter coupled transistor pair, also referred to as the xe2x80x9clower transistorsxe2x80x9d or xe2x80x9cdriver transistorsxe2x80x9d, which is coupled to a pair of cross-coupled emitter-coupled transistor pairs, also referred to as the xe2x80x9cupper transistorsxe2x80x9d, xe2x80x9cswitch transistorsxe2x80x9d or xe2x80x9cactive mixer transistorsxe2x80x9d. One set of cross-coupled, emitter-coupled transistor pairs of the upper transistors and one of the lower transistors also is referred to as a xe2x80x9clong-tailed pairxe2x80x9d. A data input, which can include an analog or digital voice and/or data input, is coupled to the emitter-coupled transistor pair. A local oscillator is coupled to the pair of cross-coupled, emitter-coupled transistor pairs, to produce a modulated output.
Gilbert Multiplier Cells are described in U.S. Pat. No. 4,156,283 to Gilbert entitled Multiplier Circuit. The Gilbert Multiplier Cell also is extensively described and analyzed in Section 10.3 of the textbook Analysis and Design of Analog Integrated Circuits by Paul Gray and Robert Meyer, John Wiley and Sons, NY, 1993, pp. 670-675, the disclosure of which is incorporated herein by reference.
A pair of Gilbert Multiplier Cells may be used to produce a quadraphase modulator, also known as an xe2x80x9cIC/IQ modulatorxe2x80x9d, an xe2x80x9cI/Q quadrature modulatorxe2x80x9d or a xe2x80x9cquadrature modulatorxe2x80x9d. Quadraphase modulators are described in U.S. Pat. No. 5,574,755, to Persico entitled I/Q Quadraphase Modulator Circuit, U.S. Pat. No. 5,530,722 to coinventor Dent, entitled Quadrature Modulator With Integrated Distributed-RC Filters and U.S. Pat. No. 5,847,623 to coinventor Hadjichristos entitled Low Noise Gilbert Multiplier Cells and Quadrature Modulators, the disclosures all of which are hereby incorporated herein by reference in their entirety.
Embodiments of the present invention can provide quadrature modulators that comprise a quadrature splitter and a pair of Gilbert Multiplier Cells coupled to the quadrature splitter that are biased in Class-B. As is well known to those having skill in the art, a Class-B circuit is biased at zero DC current so that a transistor in a Class-B stage conducts for only half of the cycle of an input sine wave. In contrast, a Class-A circuit is biased at a current that is greater than the amplitude of the signal current, so that a transistor in Class-A conducts for the entire cycle of the input signal. The quiescent current bias in the Gilbert Multiplier Cells may be substantially zero. Accordingly, reduced power consumption may be provided.
Each of the Gilbert Multiplier Cells may comprise a pair of cross-coupled, emitter-coupled transistor pairs and a driver circuit that is coupled to at least one of the emitter-coupled transistor pairs and that is biased in Class-B. The driver circuit may include at least one current mirror circuit that is coupled to at least one of the emitter-coupled transistor pairs. The driver circuit also may comprise at least one current source that selectively applies current to at least one of the emitter-coupled transistor pairs, more specifically by selectively applying current to the at least one current mirror circuit. Accordingly, the Gilbert Multiplier Cell is biased in Class-B.